I would STAY AWAY from "drive cleaning" products.
If the drive is getting full, it's almost certainly with data she's stored on it.
Does she keep a lot of movies on it?
Pics?
The only way to approach this is to:
1. find out WHAT is taking up the space
2. decide what is "tossable" and what must be saved
3. get rid of as much of the non-essential stuff as possible.
Does she use time machine?
If so, are there "local snapshots" on the hard drive?
Here's a small, free utility that can SHOW YOU what is eating up that space:
Just download it and launch it.
Go to preferences and select "show invisible files". Close preferences.
You'll see what to do next.